1). Field of the Invention
The invention concerns a measuring method and a measuring unit for determining the spatial position of a wheel rim to a measuring unit, that includes at least one camera, wherein the wheel rim lies in the field of view of the camera, as well as a wheel alignment measuring method and a wheel alignment measuring system, in which the measuring units of such kind are used.
2). Discussion of Related Art
From DE 100 43 354.5 a wheel alignment measuring system with measuring heads is known for determination of the wheel positions of the wheels of a motor vehicle on a measuring site, wherein each measuring head shows at least one camera which is aligned to a measuring target arranged in the viewing field of the camera and positioned in a fixed relation to the wheel of the motor vehicle, and wherein the picture of the camera can be evaluated by an evaluation unit in order to determine the spatial position of the measuring target and thereby of the wheel with respect to the position of the camera or of the measuring body. The unit has an optical reference system integrated in the measuring heads for calibrating the measuring heads of the wheel alignment measuring system with respect to the position of the measuring heads to each other.
Wheel alignment measuring systems with measuring devices which include two cameras which are directed under different angles on targets on the wheels, are also known from DE 197 57 760, DE 197 57 763, DE 100 32 356 and DE 100 50 653.
In these so called contact-free wheel alignment measuring systems, measuring targets must be used since the spatial position cannot be determined from features of the wheel or the wheel rim itself with sufficient accuracy through the direct picture evaluation of the pictures of the wheel or the rim.
In the DE 29 48 573, it has been attempted to bring about the spatial position of the wheel through a circle or ellipse fit on the rim horn contour. This leads to systematic errors on account of the rounding of the rim horn particularly upon turning of the steering wheel. These errors follow as a consequence that it is erroneously supposed, that the rim contour would be the perspective picture of a spatial circle, while, in reality, no spatial circle is observed in perspective. Furthermore, in a two camera construction, both the cameras of the stereo system do not see the same spatial contour, but different contours.